docs: add CIM project guide with setup instructions and common commands

Author: crossoverJie

CLAUDE.md

@@ -0,0 +1,46 @@
+# CIM Project Guide
+
+CIM (Cross-platform Instant Messaging) is a Java-based instant messaging framework.
+
+## Requirements
+
+- **Minimum JDK Version**: JDK 17
+- **Build Tool**: Maven
+- **Spring Boot Version**: 3.3.0
+
+## Environment Setup
+
+Before running compile or test commands, set the correct JDK version:
+
+```bash
+export JAVA_HOME=$JAVA_17_HOME
+```
+
+`JAVA_17_HOME` is defined in `~/.zshrc`.
+
+## Common Commands
+
+### Compile the project
+```bash
+mvn clean compile
+```
+
+### Run tests
+```bash
+mvn test
+```
+
+### Package the project
+```bash
+mvn clean package -DskipTests
+```
+
+### Full build (with tests)
+```bash
+mvn clean install
+```
+
+## Notes
+
+- Checkstyle code style checks run automatically during Maven's `validate` phase
+- Ensure code passes Checkstyle checks before committing